Device for sensing compressive forces



y 1967 A. CAPIAU DEVICE FOR SENSING COMPRESSIVE FORCES Filed June 16, 1964 lll INVENTOR. AND R E CAP l AU AGENTv United States Patent ()f" 4 Claims. in. 73-sas This invention relates to transducers, and more particularly to a device which senses compressive mechanical forces and generates a signal in response to the sensed forces.

In a compression sensing device of a known type, a block of known elastic properties is inserted between two surfaces from which compressive forces are transmitted to the block. Gages .aflixed to the block measure the shortening of the block in the direction of the applied compressive stresses. When wire strain gages are employed, the gages are placed on the block with their direction of elongation about parallel to the direction of the compressive forces. The wire gages must be placed at some distance from the load-receiving end faces of the block to avoid errors due to non-uniform strain distribution in the terminal block portions. The overall height of the block in the direction of the forces measured thus cannot be reduced beyond a certain minimum, and the known device cannot be employed where space is very limited.

The primary object of the invention is the provision of a compression sensing device whose dimension in the direction of the applied compressive forces is substantially smaller than the corresponding dimension of known devices of similar capacity.

According to a basic feature of this invention, the compression sensing device consists of two elements of which one is annular. The second element is conformingly received as a core in the central opening of the annular element and, therefore, has two opposite faces which are transverse of the axis of the opening. When a compressive force of a magnitude suflicient to cause deformation of the core element is applied to these opposite faces, the resulting radial expansion of the core element sets up circumferential strwses in the annular element. These stresses can be measured in a conventional manner, for example, by means of wire strain gages which require but a very small dimension of the device in the direction of the afore-mentioned axis.

Other features and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description of preferred embodiments, when taken in connection with the appended drawing in which:

FIG. 1 shows a first compression sensing device of the invention in a perspective view, a portion of the device being cut away to reveal internal structure; and

FIG. 2 illustrates a modified compression sensing device in a view corresponding to that of FIG. 1.

Referring now to the drawing in detail, and initially to FIG. 1, there is seen a steel ring 1 of generally rectangular cross section. The central opening of the ring 1 is almost completely occupied by a disk-shaped rubber core 2 which conformingly engages the ring 1. In the illustrated relaxed condition, the core 2 projects axially beyond the radial planes defined by the end faces of the ring 1. Circular grooves at the edges of the core 2 about the common axis of the ring 1 and the core 2 extend axially slightly inward of the corresponding end faces of the ring 1, and hold restraining rings 3 and 4 which are spiral springs coiled about respective steel reinforcing rods 31, 41. Thus, as is shown in the drawing, the core 2 is of the same general thickness as the ring 1 and is actually Patented May 16, 1967 only slightly greater in thickness than the ring 1 so that, in the relaxed condition thereof, the opposite faces of the core 2 are situated slightly beyond the opposite end faces of the ring 1.

The outer cylindrical face of the ring 1 is spacedly enveloped by a protective casing 5 which engages the peripheral portions of the end faces and thereby encloses a protected annular space about the circumference of the ring 1. Strain wire gages, conventional in themselves and not visible in the drawing, are arranged in the protected space. Two of the gages are arranged to measure the circumferential dimensional changes of the ring 1, and two compensating gages are arranged at right angles to the first-mentioned gages, as is usual with a conventional bridge arrangement. The strain gaging arrangement is represented in the drawing only by four leads 6 which emerge from an opening in the casing 5 in sealing engage- .ment with the latter. The electrical signal received from the leads 6 is suitably modified and may further be amplified to drive an indicating, recording, or controlling instrument, as is well known.

The afore-described device is operated as follows:

It is placed between two flat surfaces which apply compressive forces to the parallel opposite faces of the rubber core, 2, but do not touch the steel ring 1. The rubber core behaves mechanically like a viscous fluid, and the compressive stresses received are transmitted as radial forces to the ring 1. The resulting circumferential deformation of the ring is detected by the strain gages and converted into an electrical signal. This signal is evaluated by means of a calibration curve or chart which is established by subjecting the device to compressive stresses of known magnitude.

The restraining rings 3, 4 limit or prevent radial expansion of the axially projecting portions of the rubber ring 2 under the applied forces which may otherwise cause extrusion of the rubber between the faces of the steel ring 1 and the applied fiat surfaces (not shown) from which compressive forces are transmitted to the rubber core 2.

The axial length of the compression sensing device of the invention may be very small when compared to known devices of similar load capacity. A typical compression sensing device of the invention having a useful measuring range from 0 to 2,550 Newtons has an outer diameter of 15 mm. and an axial height of only 8 mm. A typical device of the invention having a range of 0 to 500,000 Newtons has a diameter of mm. and a height of 10 mm.

The modified compression sensing device illustrated in FIG. 2 has a steel ring 11 closely similar to the aforedescribed ring 1. Its axial opening is partly occupied by a rubber ring 12 of cross-shaped cross section which conformingly engages the inner axial wall of the steel ring 11. The axial thickness of the rubber ring 12 in the illustrated relaxed condition is also substantially the same as and only somewhat greater than that of the steel ring 11. The four grooves at its edges receive four restraining rings 13, 14, 18, 19, each consisting of a spiral spring having a central reinforcing steel rod, as described with references to the restraining rings 3, 4.

A protective casing 15 envelopes the circumference of the ring 11 and of the attached wire strain gages which are not visible in the view of FIG. 2, and from which leads 16 emerge through the casing 15.

The rubber ring 12 is reinforced by a backing ring 17 of steel which is coaxial with the rings 11 and 12. The backing ring 1'7 is axially shorter than the rubber ring 12 and conformingly engages the inner axial surface of the rubber ring. The four restraining rings 13, 14, 18, 19 prevent radial extrusion of the rubber ring 12 in an inward as well as an outward direction through an axial "J 1%) gap between the transverse end faces of the rings 11 and 17 and the cooperating flat surfaces (not shown) from which compressive forces are transmitted to the rubber ring 12.

The field of application of compression sensing devices in general is too well known to require more detailed description. The specific embodiment illustrated in FIG. 2 is particularly useful in measuring pressure exerted by nuts and in similar applications in which a screw or the like may pass through the central aperture in the ring 17.

Many modifications of the invention will readily suggest themselves to those skilled in the art in the light of the above teachings. A wide range of materials of construction other than the steel and rubber specifically mentioned with reference to the drawing may be employed without departing from the spirit and scope of the inven tion. If the device is to be employed more than once, it is necessary that the outer annular member 1, 11 be of elastically resilient material, and that it be employed under conditions in which the circumferential stresses remain Well below the elastic limit of the material.

The limitations on the nature of the weaker core member 2, 12 are even less stringent, and a core member of virtually any material capable of being deformed by the applied compressive stresses while conformingly received in the central opening of the annular member 1, 11 is operative. While the use of an eiastomeric material, such as natural or synthetic rubber, offers obvious advantages, other solid materials such as lead, can be used, and viscous liquids may be employed under conditions which make leakage losses insignificant.

Obviously, many other modifications and variations of the present invention are possible. t is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A compression sensing device comprising, in combination:

(a) an annular member of elastic material defining an opening passing therethrough in a predetermined direction and having opposed end faces;

(b) a deformable member received in said opening and conformingly engaging said annular member,

(1) said deformable member having, in a relaxed condition, two parallel opposite faces transverse of said direction and situated only slightly beyond respective ones of said end faces of said annular member,

(2) whereby a compressive force of a magnitude SllfilCiBflt to deform said deformable member and applied to said faces causes a circumferen- ,tial stress in said annular member;

(c) signal generating means for generating a signal responsive to the magnitude of said circumferential stress; and

(d) restraining means for limiting expansion of said deformable member in the region of said parallel end faces thereof transversely of said direction under said applied force, said restraining means being sur rounded by and engaging said annular member and surrounding said deformable member in the region of said parallel end faces thereof.

2. A device as set forth in claim 1, wherein said deformable member is of elastomeric material.

3. A device as set forth in claim 1, wherein said signal generating means include strain gage means, the device further comprising aprotective casing enclosing respective portions of said strain gage means and of said annular member.

4. A compression sensing device comprising, in combination:

(a) an annular member of elastic material defining an opening passing therethrough in a predetermined direction and having opposed end faces; 7

(b) a deformable member formed with an opening therethrough in said direction, the deformable memher being received in the openingof the annular member and conformingly engaging said annular member,

(1) said deformable member having, in a relaxed condition, two parallel. opposite faces transverse of said direction and situated only slightly beyond respective ones of said end faces of said annular member,

(2) whereby a compressive force of a magnitude sufiicient to deform said deformable member and applied to said faces causes a circumferential stress in said annular member;

(0) signal generating means for generating a signal responsive to the magnitude of said circumferential stress; and

(d) a backing member more resistant to deformation than said deformable member and received in said opening in the deformable member in conforming engagement with said deformable member.

References Cited by the Examiner UNITED STATES PATENTS 3,139,598 6/1964 Ruge 7388.5 X 3,153,772 10/1964 Dorr.

3,199,057 8/1965 Gindes et al. 73-885 X RICHARD C. QUEISSER, Primary Examiner.

J. .l. GILL, Assistant Examiner. 

1. A COMPRESSION SENSING DEVICE COMPRISING, IN COMBINATION: (A) AN ANNULAR MEMBER OF ELASTIC MATERIAL DEFINING AN OPENING PASSING THERETHROUGH IN A PREDETERMINED DIRECTION AND HAVING OPPOSED END FACES; (B) A DEFORMABLE MEMBER RECEIVED IN SAID OPENING AND CONFORMINGLY ENGAGING SAID ANNULAR MEMBER, (1) SAID DEFORMABLE MEMBER HAVING, IN A RELAXED CONDITION, TWO PARALLEL OPPOSITE FACES TRANSVERSE OF SAID DIRECTION AND SITUATED ONLY SLIGHTLY BEYOND RESPECTIVE ONES OF SAID END FACES OF SAID ANNULAR MEMBER, (2) WHEREBY A COMPRESSIVE FORCE OF A MAGNITUDE SUFFICIENT TO DEFORM SAID DEFORMABLE MEMBER AND APPLIED TO SAID FACES CAUSES A CIRCUMFERENTIAL STRESS IN SAID ANNULAR MEMBER; (C) SIGNAL GENERATING MEANS FOR GENERATING A SIGNAL RESPONSIVE TO THE MAGNITUDE OF SAID CIRCUMFERENTIAL STRESS; AND (D) RESTRAINING MEANS FOR LIMITING EXPANSION OF SAID DEFORMABLE MEMBER IN THE REGION OF SAID PARALLEL END FACES THEREOF TRANSVERSELY OF SAID DIRECTION UNDER SAID APPLIED FORCE, SAID RESTRAINING MEANS BEING SURROUNDED BY AND ENGAGING SAID ANNULAR MEMBER AND SURROUNDING DEFORMABLE MEMBER IN THE REGION OF SAID PARALLEL END FACES THEREOF. 